1. Field of the Invention:
This invention relates to an automatic sample inlet system for a mass spectrometer or the like. More specifically, the invention relates to an improved sample inlet system for a mass spectrometer wherein a series of samples are sequentially transferred from a room temperature vacuum environment into a sample vaporization oven and subsequently injected into a mass spectrometer which operates on a preprogrammed automated basis.
2. Description of the Prior Art:
The introduction or injection of a small sample of vapors of a high molecular weight compound into various scientific apparatus, equipment, analytical instruments, or the like has historically presented a serious problem. In order to volatilize such a sample, high temperatures and low pressures are required. Such conditions inherently imply heating and cooling with an associated thermal expansion problem as well as sample stability or sample environment compatibility considerations. Thus, in designing and building an acceptable sample inlet system, particularly when the system is to be used repeatedly such as when a series of samples are to be injected into a mass spectrometer, one must account for repeated expansion and contraction of the sample ampoule seal.
One of the earliest designs for a mass spectrometer inlet system which required high temperature vaporization of the sample was a gallium covered glass frit inlet. Later, a gallium orifice inlet was introduced that eliminated the problem of sample fractionation caused by the frit, but had an upper temperature limit of about 300.degree. C. and retained the property of potential catalytic reaction by contact of the sample with hot gallium. Various all glass inlet system have also been described in scientific literature. In one system a weighed sample sealed in a capillary tube is introduced into an expansion vessel and then broken with a magnetic plunger. Another all glass heated mass spectrometer inlet which extends the upper operating temperature from about 300.degree. to 450.degree. C. for very small samples (0.1 milligrams) is disclosed in J. Mass Spectrometry and Ion Physics (1968), pages 87-92 and is the subject matter of U.S. Pat. No. 3,594,574. This inlet system includes a valved manifold and a vacuum lock for sealingly connecting a sample container to the manifold. The lock chamber is formed by two concentric tubes joined at one end by a ring seal and polished to an optical flat mating surface at the other end. A vacuum outlet tube is connected to the annular space or central chamber between the concentric tubes such that the sample ampoule tube with optical flat mating surface can be held against this vacuum lock during operation. This optical flat vacuum lock inlet system has proved to be a reliable method of introducing small samples at higher temperatures without loss of vacuum in the mass spectrometer, yet is limited in that the presence of the concentric tubes create dissimilar rates of thermal expansion upon heating.
In a recent U.S. Pat. No. 4,465,930, a specific improvement in this glass inlet system is disclosed wherein the interface between the two separable polished glass flats is provided with a groove that is substantially surrounded by an essentially continuous matrix of glass and connected to a vacuum source during operation. In this manner, the sample lock exhibits greater stability and more uniform expansion and contraction during heat up or cool down. In principle, it would be ideal to be able to employ such an improved vacuum lock in an automated glass inlet system such that a preprogrammed sequence of samples could be introduced into the mass spectrometer without an operator being continuously present to perform the manipulative steps necessary for injecting individual samples. The present invention is viewed as being such a system.